The present invention relates to variable displacement pumps having reciprocatable vanes.
Vane pumps are well known. In a very simple form, a pumping rotor carries vanes around a casing having a cylindrical volume which is eccentric to the center of rotation of the rotor. This eccentricity results in the volume between vanes changing cyclically. With ports in the casing positioned appropriately, the changing volume between vanes can cause pumping. In a known rotary pump, the extent of eccentricity between rotor and casing can be changed to alter the pumping displacement.
Another known pump employs a rotor having along its periphery axial slots carrying vanes which are axially reciprocatable within the slots. A coaxial cam in the shape of a truncated cylinder lies alongside these vanes and directs their axial reciprocation at the rate of one cycle per revolution of the rotor. While this arrangement can be set to change the phasing of the vanes and the associated volume between them, this phasing change has some disadvantages. When the phasing is changed to reduce pump displacement, the fluid being pumped is pressurized and depressurized non-productively, thereby increasing the load on the various moving parts and pump bearings.
Another known fluid motor or pump has a duplex construction. This construction includes two cams on opposite sides of a blade carrier, forming two working chambers. The blade carrier has two circumferential series of separate blades. However, this known device does not provide for relative rotation between the different cams to change the displacement of a pump. Thus this design suffers the unnecessary loading mentioned previously.
It is also known to alter the displacement of a vane pump by distorting the surface of its cam. A disadvantage with such distortions is that the peak acceleration and thus the forces applied to the vane varies significantly as the cam is distorted. Thus vane wear increases as the cam is distorted to non-ideal configurations.
Accordingly, there is a need for a variable displacement vane pump which has a cam surface that does not create undue acceleration and stress. The displacement of this pump should be variable without inducing nonproductive compression and decompression of fluids that can cause unnecessary stress and wear. Furthermore the pump ought to be simple, efficient and reliable.